As per Relevance of the word tunneling, we have this rfc below:
Network Working Group E.
Request for Comments: 3077
Category: Standards Track W.
INRIA Sophia-
H.
N.
Y.
March 2001
A Link-Layer Tunneling Mechanism for Unidirectional
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
Copyright
Copyright (C) The Internet Society (2001). All Rights Reserved
This document describes a mechanism to emulate full
connectivity between all nodes that are directly connected by
unidirectional link. The "receiver" uses a link-layer
mechanism to forward datagrams to "feeds" over a
bidirectional IP (Internet Protocol) network. As it is
at the link-layer, protocols in addition to IP may also be
by this mechanism
1.
Internet routing and upper layer protocols assume that links
bidirectional, i.e., directly connected hosts can communicate
each other over the same link
This document describes a link-layer tunneling mechanism that
a set of nodes (feeds and receivers, see Section 2 for terminology
which are directly connected by a unidirectional link to
datagrams as if they were all connected by a bidirectional link.
present a generic topology in section 3 with a tunneling
Duros, et al. Standards Track [Page 1]
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that supports multiple feeds and receivers. Note, this mechanism
not designed for topologies where a pair of nodes are connected by 2
unidirectional links in opposite direction
The tunneling mechanism requires that all nodes have an
interface to an IP interconnected infrastructure
The tunneling mechanism is implemented at the link-layer of
interface of every node connected to the unidirectional link.
aim is to hide from higher layers, i.e., the network layer and above
the unidirectional nature of the link. The tunneling mechanism
includes an automatic tunnel configuration protocol that allows
to come up/down at any time
Generic Routing Encapsulation [RFC2784] is suggested as the
mechanism as it provides a means for carrying IP, ARP datagrams,
any other layer-3 protocol between nodes
The tunneling mechanism described in this document was discussed
agreed upon by the UDLR working group
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD
SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in
document, are to be interpreted as described in [RFC2119].
2.
Unidirectional link (UDL): A one way transmission link, e.g.,
broadcast satellite link
Receiver: A router or a host that has receive-only connectivity to
UDL
Send-only feed: A router that has send-only connectivity to a UDL
Receive capable feed: A router that has send-and-receive
to a UDL
Feed: A send-only or a receive capable feed
Node: A receiver or a feed
Bidirectional interface: a typical communication interface that
send or receive packets, such as an Ethernet card, a modem, etc
Duros, et al. Standards Track [Page 2]
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3.
Feeds and receivers are connected via a unidirectional link. Send
only feeds can only send data over this unidirectional link,
receivers can only receive data from it. Receive capable feeds
both send and receive capabilities
This mechanism has been designed to work with any topology with
number of receivers and one or more feeds. However, it is
that the number of feeds will be small. In particular, the
case of a single send-only feed and multiple receivers is among
topologies supported
A receiver has several interfaces, a receive-only interface and
or more additional bidirectional communication interfaces
A feed has several interfaces, a send-only or a send-and-
capable interface connected to the unidirectional link and one
more additional bidirectional communication interfaces. A feed
be a router
Tunnels are constructed between the bidirectional interfaces
nodes, so these interfaces must be interconnected by an
infrastructure. In this document we assume that that
is the Internet
Figure 1 depicts a generic topology with several feeds and
receivers
Unidirectional
---->---------->------------------->------
| | | |
|f1u |f2u |r2u |r1
-------- -------- -------- -------- ----------
|Feed 1| |Feed 2| |Recv 2| |Recv 1|---|subnet A
-------- -------- -------- -------- ----------
|f1b |f2b |r2b |r1b |
| | | | |
----------------------------------------------------
| Internet |
----------------------------------------------------
Figure 1: Generic
f1u (resp. f2u) is the IP address of the 'Feed 1' (resp. Feed 2)
send-only interface
Duros, et al. Standards Track [Page 3]
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f1b (resp. f2b) is the IP address of the 'Feed 1' (resp. Feed 2)
bidirectional interface connected to the Internet
r1u (resp. r2u) is the IP address of the 'Receiver 1' (resp.
2) receive-only interface
r1b (resp. r2b) is the IP address of the 'Receiver 1' (resp.
2) bidirectional interface connected to the Internet
Subnet A is a local area network connected to recv1.
Note that nodes have IP addresses on their unidirectional and
bidirectional interfaces. The addresses on the
interfaces (f1u, f2u, r1u, r2u) will be drawn from the same
network. In general the addresses on the bidirectional
(f1b, f2b, r1b, r2b) will be drawn from different IP networks,
the Internet will route between them
4. Problems related to unidirectional
Receive-only interfaces are "dumb" and send-only interfaces
"deaf". Thus a datagram passed to the link-layer driver of
receive-only interface is simply discarded. The link-layer of
send-only interface never receives anything
The network layer has no knowledge of the underlying
technology except that it considers its access as bidirectional
Basically, for outgoing datagrams, the network layer selects
correct first hop on the connected network according to a
table and passes the packet(s) to the appropriate link-layer driver
Referring to Figure 1, Recv 1 and Feed 1 belong to the same network
However, if Recv 1 initiates a 'ping f1u', it cannot get a
from Feed 1. The network layer of Recv 1 delivers the packet to
driver of the receive-only interface, which obviously cannot send
to the feed
Many protocols in the Internet assume that links are bidirectional
In particular, routing protocols used by directly connected
no longer behave properly in the presence of a unidirectional link
5. Emulating a broadcast bidirectional
The simplest solution is to emulate a broadcast capable link-
network. This will allow the immediate deployment of existing
level protocols without change. Though other network structures
such as NBMA, could also be emulated, a broadcast network is
generally useful. Though a layer 3 network could be emulated,
Duros, et al. Standards Track [Page 4]
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link-layer network allows the immediate use of any other
layer protocols, and most particularly allows the immediate use
ARP
A link-layer tunneling mechanism which emulates
connectivity in the presence of a unidirectional link will
described in the next Section. We first consider the
communication scenarios which characterize a broadcast network
order to define what functionalities the link-layer
mechanism has to perform in order to emulate a
broadcast link
Here we enumerate the scenarios which would be feasible on
broadcast network, i.e., if feeds and receivers were connected by
bidirectional broadcast link
Scenario 1: A receiver can send a packet to a feed (point-to-
communication between a receiver and a feed).
Scenario 2: A receiver can send a broadcast/multicast packet on
link to all nodes (point-to-multipoint).
Scenario 3: A receiver can send a packet to another receiver (point
to-point communication between two receivers).
Scenario 4: A feed can send a packet to a send-only feed (point-to
point communication between two feeds).
Scenario 5: A feed can send a broadcast/multicast packet on the
to all nodes (point-to-multipoint).
Scenario 6: A feed can send a packet to a receiver or a
capable feed (point-to-point).
These scenarios are possible on a broadcast network. Scenario 6
already feasible on the unidirectional link. The link-
tunneling mechanism should therefore provide the functionality
support scenarios 1 to 5.
Note that regular IP forwarding over such an emulated network (i.e.,
using the emulated network as a transit network) works correctly;
next hop address at the receiver will be the unidirectional
address of another router (a feed or a receiver) which will
relay the packet
Duros, et al. Standards Track [Page 5]
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6. Link-layer tunneling
This link-layer tunneling mechanism operates underneath the
layer. Its aim is to emulate bidirectional link-layer connectivity
This is transparent to the network layer: the link appears
behaves to the network layer as if it was bidirectional
Figure 2 depicts a layered representation of the link-layer
mechanism in the case of Scenario 1.
Send-only Feed
decapsulation
/-----***************----\ /-->---***************--\
| | | |
| | | |
--|---------------------- | | ---------------------|---
| | f1b | f1u | | | | x r1u | r1b | |
| | | ^ | | IP | | | | v |
| ^ | | | v | | | | | |
| | | | | | | | v | | |
|-|---------|-------|---| | | |----|------|--------|--|
| | | | | | ^ | | | | |
| | | | | | LL | | | | | |
| | | | | | | | | | | |
| | | O------/ \<------O | | |
|-|---------|-----------| |-----------|--------|--|
| | | | | | | |
| | | | PHY | | | |
| | | | | | v |
| | | | | | | | | |
--|-----------|---------- ----------|----------|---
| Bidir | Send-Only Recv-Only | Bidir |
^ Interf | Interf UDL Interf | Interf |
| \------------>------->------------/ |
\----------------------<------------------------<--------/
Bidirectional
x : IP layer at the receiver generates a datagram to be
on the receive-only interface
O : Entry point where the link-layer tunneling mechanism
triggered
Figure 2: Scenario 1 using the link-layer Tunneling
Duros, et al. Standards Track [Page 6]
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6.1. Tunneling mechanism on the
On the receiver, a datagram is delivered to the link-layer of
unidirectional interface for transmission (see Figure 2). It is
encapsulated within a MAC header corresponding to the
link. This packet cannot be sent directly over the link, so it
then processed by the tunneling mechanism
The packet is encapsulated within an IP header whose destination
the IP address of a feed bidirectional interface (f1b or f2b).
destination address is also called the tunnel end-point.
mechanism for a receiver to learn these addresses and to choose
feed is explained in Section 7. The type of encapsulation
described in Section 8.
In all cases the packet is encapsulated, but the tunnel end-point (
IP address) depends on the encapsulated packet's destination
address. If the destination MAC address is
1) the MAC address of a feed interface connected to
unidirectional link (Scenario 1). The datagram
encapsulated, the destination address of the
datagram is the feed tunnel end-point (f1b referring to
2).
2) a MAC broadcast/multicast address (Scenario 2). The
is encapsulated, the destination address of the
datagram is the default feed tunnel end-point. See Section 7.4
for further details on the default feed
3) a MAC address that belongs to the unidirectional network but
not a feed address (Scenario 3). The datagram is encapsulated
the destination address of the encapsulating datagram is
default feed tunnel end-point
The encapsulated datagram is passed to the network layer
forwards it according to its destination address. The
address is a feed bidirectional interface which is reachable via
Internet. In this case, the encapsulated datagram is forwarded
the receiver bidirectional interface (r1b).
6.2. Tunneling mechanism on the
A feed processes unidirectional link related packets in two
ways
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- packets generated by a local application or packets routed
usual by the IP layer may have to be forwarded over
unidirectional link (Section 6.2.1)
- encapsulated packets received from another receiver or feed
tunnel processing (Section 6.2.2).
A feed cannot directly send a packet to a send-only feed over
unidirectional link (Scenario 4). In order to emulate this type
communication, feeds have to tunnel packets to send-only feeds.
feed MUST maintain a list of all other feed tunnel end-points.
list MUST indicate which are send-only feed tunnel end-points.
is configured manually at the feed by the local administrator,
described in Section 7.
6.2.1. Forwarding packets over the unidirectional
When a datagram is delivered to the link-layer of the
interface of a feed for transmission, its treatment depends on
packet's destination MAC address. If the destination MAC address is
1) the MAC address of a receiver or a receive capable
(Scenario 6). The packet is sent over the unidirectional link
This is classical "forwarding".
2) the MAC address of a send-only feed (Scenario 4). The
is encapsulated and sent to the send-only feed tunnel end
point. The type of encapsulation is described in Section 8.
3) a broadcast/multicast destination (Scenario 5). The packet
sent over the unidirectional link. Concurrently, a copy
this packet is encapsulated and sent to every feed of the
of send-only feed tunnel end-points. Thus
broadcast/multicast will reach all receivers and all send-
feeds
6.2.2. Receiving encapsulated
Feeds listen for incoming encapsulated datagrams on their
end-points. Encapsulated packets will have been received on
bidirectional interface, and traversed their way up the IP stack
They will then enter a decapsulation process (See Figure 2).
Decapsulation reveals the original link-layer packet. Note that
has not been modified in any way by intermediate routers;
particular, the original MAC header will be intact
Duros, et al. Standards Track [Page 8]
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Further actions depend on the destination MAC address of the link
layer packet, which can be
1) the MAC address of the feed interface connected to
unidirectional link, i.e., own MAC address (Scenarios 1 and 4).
The packet is passed to the link-layer of the
connected to the unidirectional link which can then deliver
up to higher layers. As a result, the datagram is processed
if it was coming from the unidirectional link, and
delivered locally. Scenarios 1 and 4 are now feasible,
receiver or a feed can send a packet to a feed
2) a receiver address (Scenario 3). The packet is passed to
link-layer of the interface connected to the
link. It is directly sent over the unidirectional link, to
indicated receiver. Note, the packet must not be
locally. Scenario 3 is now feasible, a receiver can send
packet to another receiver
3) a broadcast/multicast address, this corresponds to Scenarios 2
and 5. We have to distinguish two cases, either (i)
encapsulated packet was sent from a receiver or (ii) from
feed (encapsulated broadcast/multicast packet sent to a send
only feed). These cases are distinguished by examining
source address of the encapsulating packet and comparing
with the configured list of feed IP addresses. The action
taken is
i) the feed was designated as a default feed by a receiver
forward the broadcast/multicast packet. The feed is then
charge of sending the multicast packet to all nodes
Delivery to all nodes is accomplished by executing all 3
the following actions
- The packet is encapsulated and sent to the list of send
only feed tunnel end-points
- Also, the packet is passed to the link-layer of
interface which forwards it directly over
unidirectional link (all receivers and receive
feeds receive it).
- Also, the link-layer delivers it locally to
layers
Caution: a receiver which sends an
broadcast/multicast packet to a default feed will
its own packet via the unidirectional link.
filtering as described in [RFC1112] must be applied
Duros, et al. Standards Track [Page 9]
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ii) the feed receives the packet and keeps it for
delivery. The packet is passed to the link-layer of
interface connected to the unidirectional link
delivers it to higher layers
Scenario 2 is now feasible, a receiver can send
broadcast/multicast packet over the unidirectional link and
will be heard by all nodes
7. Dynamic Tunnel Configuration Protocol (DTCP
Receivers and feeds have to know the feed tunnel end-points in
to forward encapsulated datagrams (e.g., Scenarios 1 and 4).
The number of feeds is expected to be relatively small (Section 3),
so at every feed the list of all feeds is configured manually.
list should note which are send-only feeds, and which are
capable feeds. The administrator sets up tunnels to all send-
feeds. A tunnel end-point is an IP address of a bidirectional
on a send-only feed
For scalability reasons, manual configuration cannot be done at
receivers. Tunnels must be configured and maintained dynamically
receivers, both for scalability, and in order to cope with
following events
1) New feed detection
When a new feed comes up, every receiver must create a
to enable bidirectional communication with it
2) Loss of unidirectional link detection
When the unidirectional link is down, receivers must
their tunnels. The tunneling mechanism emulates
connectivity between nodes. Therefore, if the
link is down, a feed should not receive datagrams from
receivers. Protocols that consider a link as operational
they receive datagrams from it (e.g., the RIP
[RFC2453]) require this behavior for correct operation
3) Loss of feed detection
When a feed is down, receivers must disable their
tunnel. This prevents unnecessary datagrams from
tunneled which might overload the Internet. For instance
there is no need for receivers to forward a broadcast
through a tunnel whose end-point is down
Duros, et al. Standards Track [Page 10]
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The DTCP protocol provides a means for receivers to
discover the presence of feeds and to maintain a list of
tunnel end-points. Feeds periodically announce their tunnel end
point addresses over the unidirectional link. Receivers listen
these announcements and maintain a list of tunnel end-points
7.1. The HELLO
The DTCP protocol is a 'unidirectional protocol', messages are
sent from feeds to receivers
The packet format is shown in Figure 3. Fields contain
integers, in normal Internet order with the most significant
first. Each tick mark represents one bit
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Vers | Com | Interval | Sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| res |F|IP Vers| Tunnel Type | Nb of FBIP | reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Feed BDL IP addr (FBIP1) (32/128 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ..... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Feed BDL IP addr (FBIPn) (32/128 bits) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: Packet
Every datagram contains the following fields, note that constants
written in uppercase and are defined in Section 7.5:
Vers (4 bit unsigned integer): DTCP version number. MUST
DTCP_VERSION
Com (4 bit unsigned integer): Command field, possible values
1 - JOIN A message announcing that the feed sending this
is up and running
2 - LEAVE A message announcing that the feed sending this
is being shut down
Interval (8 bit unsigned integer): Interval in seconds between
messages for the IP protocol in "IP Vers". Must be > 0.
recommended value is HELLO_INTERVAL. If this value is increased
the feed MUST continue to send HELLO messages at the old rate
at least the old HELLO_LEAVE period
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Sequence (16 bit unsigned integer): Random value initialized at
time and incremented by 1 every time a value of the HELLO
is modified
res (3 bits): Reserved/unused field, MUST be zero
F (1 bit): bit indicating the type of feed
0 = Send-only
1 = Receive-capable
IP Vers (4 bit unsigned integer): IP protocol version of the
bidirectional IP addresses (FBIP):
4 = IP version 4
6 = IP version 6
Tunnel Type (8 bit unsigned integer): tunneling protocol supported
the feed. This value is the IP protocol number defined
[RFC1700] [iana/protocol-numbers] and their
descendents. Receivers MUST use this form of tunnel
when tunneling to the feed
47 = GRE [RFC2784] (recommended
Other protocol types allowing link-layer encapsulation
permitted. Obtaining new values is documented in [RFC2780].
Nb of FBIP (8 bit unsigned integer): Number of bidirectional IP
addresses which are enumerated in the HELLO
reserved (8 bits): Reserved/unused field, MUST be zero
Feed BDL IP addr (32 or 128 bits). The bidirectional IP address
is the IP address of a feed bidirectional interface (tunnel end
point) reachable via the Internet. A feed has 'Nb of FBIP'
addresses which are operational tunnel end-points. They
enumerated in preferred order. FBIP1 being the most
tunnel end-point
7.2. DTCP on the feed: sending HELLO
The DTCP protocol runs on top of UDP. Packets are sent to the "
announcement" multicast address over the unidirectional link on
HELLO_PORT with a TTL of 1. Due to existing deployments a
SHOULD also support the use of the old DTCP announcement address,
described in Appendix B
The source address of the HELLO packet is set to the IP address
the feed interface connected to the unidirectional link. In the
of the document, this value is called FUIP (Feed Unidirectional
address).
Duros, et al. Standards Track [Page 12]
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The process in charge of sending HELLO packets fills every field
the datagram according to the description given in Section 7.1.
As long as a feed is up and running, it periodically announces
presence to receivers. It MUST send HELLO packets containing a
command every HELLO_INTERVAL over the unidirectional link
Referring to Figure 1 in Section 3, Feed 1 (resp. Feed 2) sends
messages with the FBIP1 field set to f1b (resp. f2b).
When a feed is about to be shut down, or when routing over
unidirectional link is about to be intentionally interrupted, it
recommended that feeds
1) stop sending HELLO messages containing a JOIN command
2) send a HELLO message containing a LEAVE command to
receivers that the feed is no longer performing routing
the unidirectional link
7.3. DTCP on the receiver: receiving HELLO
Based on the reception of HELLO messages, receivers discover
presence of feeds, maintain a list of active feeds, and keep track
the tunnel end-points for those feeds
For each active feed, and each IP protocol supported, at least
following information will be kept
FUIP - feed unidirectional link IP
FUMAC - MAC address corresponding to the above
(FBIP1,...,FBIPn) - list of tunnel end-
tunnel type - tunnel type supported by this
Sequence - "Sequence" value from the last HELLO
from this
timer - used to timeout this
The FUMAC value for an active feed is needed for the operation
this protocol. However, the method of discovery of this value is
specified here
Initially, the list of active feeds is empty
When a receiver is started, it MUST run a process which joins
"DTCP announcement" multicast group and listens to incoming
on the HELLO_PORT port from the unidirectional link
Duros, et al. Standards Track [Page 13]
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Upon the reception of a HELLO message, the process checks the
number of the protocol. If it is different from HELLO_VERSION,
packet is discarded and the process waits for the next
packet
After successfully checking the version number further action
on the type of command
- JOIN
The process verifies if the address FUIP already belongs to
list of active feeds
If it does not, a new entry, for feed FUIP, is created and
to the list of active feeds. The number of feed bidirectional
addresses to read is deduced from the 'Nb of FBID' field.
tunnel end-points (FBIP1,...,FBIPn) can then be added to the
entry. The tunnel Type and Sequence values are also taken
the HELLO packet and recorded in the new entry. A timer set
HELLO_LEAVE is associated with this entry
If it does, the sequence number is compared to the sequence
contained in the previous HELLO packet sent by this feed. If
are equal, the timer associated with this entry is reset
HELLO_LEAVE. Otherwise all the information corresponding to
is set to the values from the HELLO packet
Referring to Figure 1 in Section 3, both receivers (recv 1
recv 2) have a list of active feeds containing two entries: Feed 1
with a FUIP of f1u and a list of tunnel end-points (f1b); and
2 with a FUIP of f2u and a list of tunnel end-points (f2b).
- LEAVE
The process checks if there is an entry for FUIP in the list
active feeds. If there is, the timer is disabled and the entry
deleted from the list. The LEAVE message provides a means
quickly updating the list of active feeds
A timeout occurs for either of two reasons
1) a feed went down without sending a LEAVE message. As
messages are no longer sent from this feed, a timeout occurs
HELLO_LEAVE after the last JOIN message
2) the unidirectional link is down. Thus no more JOIN
are received from any of the feeds, and they will each
independently. The timeout of each entry depends on
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individual HELLO_LEAVE value, and when the last JOIN
was sent by that feed, before the unidirectional link
down
In either case, bidirectional connectivity can no longer be
between the receiver and the feed (FUIP): either the feed is
longer routing datagrams over the unidirectional link, or the link
down. Thus the associated entry is removed from the list of
feeds, whatever the cause. As a result, the list only
operational tunnel end-points
The HELLO protocol provides receivers with a list of feeds, and
list of usable tunnel end-points (FBIP1,..., FBIPn) for each feed
In the following Section, we describe how to integrate the
protocol into the tunneling mechanism described in Sections 6.1
6.2.
7.4. Tunneling mechanism using the list of active
This Section explains how the tunneling mechanism uses the list
active feeds to handle datagrams which are to be tunneled.
to Section 6.1, it shows how feed tunnel end-points are selected
The choice of the default feed is made independently at
receiver. The choice is a matter of local policy, and this policy
out of scope for this document. However, as an example, the
feed may be the feed that has the lowest round trip time to
receiver
When a receiver sends a packet to a feed, it must choose a
end-point from within the FBIP list. The 'preferred FBIP'
generally FBIP1 (Section 7.1). For various reasons, a receiver
decide to use a different FBIP, say FBIPi instead of FBIP1, as
tunnel end-point. For example, the receiver may have
connectivity to FBIPi. This decision is taken by the
administrator
Here we show how the list of active feeds is involved when a
tunnels a link-layer packet. Section 6.1 listed the following cases
depending on whether the MAC destination address of the packet is
1) the MAC address of a feed interface connected to
unidirectional link: This is TRUE if the address matches
FUMAC address in the list of active feeds. The packet
tunneled to the preferred FBIP of the matching feed
2) the broadcast address of the unidirectional link or a
address
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This is determined by the MAC address format rules, and
list of active feeds is not involved. The packet is
to the preferred FBIP of the default feed
3) an address that belongs to the unidirectional network but
not a feed address
This is TRUE if the address is neither broadcast nor multicast
nor found in the list of active feeds. The packet is
to the preferred FBIP of the default feed
In all cases, the encapsulation type depends on the tunnel
required by the feed which is selected
7.5. Constant
DTCP_VERSION is 1.
HELLO_INTERVAL is 5 seconds
"DTCP announcement" multicast group is 224.0.0.36, assigned by IANA
HELLO_PORT is 652. It is a reserved system port assigned by IANA,
other traffic must be allowed
HELLO_LEAVE is 3*Interval, as advertised in a HELLO packet, i.e., 15
seconds if the default HELLO_INTERVAL was advertised
8. Tunnel encapsulation
The tunneling mechanism operates at the link-layer and
bidirectional connectivity amongst receivers and feeds. We
that hardware connected to the unidirectional link supports
and unicast MAC addressing. That is, a feed can send a packet to
particular receiver using a unicast MAC destination address or to
set of receivers using a broadcast/multicast destination address
The hardware (or the driver) of the receiver can then filter
incoming packets sent over the unidirectional links without
assumption about the encapsulated data type
In a similar way, a receiver should be capable of sending unicast
broadcast MAC packets via its tunnels. Link-layer packets
encapsulated. As a result, after decapsulating an incoming packet
the feed can perform link-layer filtering as if the data
directly from the unidirectional link (See Figure 2).
Generic Routing Encapsulation (GRE) [RFC2784] suits our
because it specifies a protocol for encapsulating arbitrary packets
and allows use of IP as the delivery protocol
Duros, et al. Standards Track [Page 16]
RFC 3077 LL Tunneling Mechanism for UDLs March 2001
The feed's local administrator decides what encapsulation it
demand that receivers use, and sets the tunnel type field in
HELLO message appropriately. The value 47 (decimal) indicates GRE
Other values can be used, but their interpretation must be
upon between feeds and receivers. Such usage is not defined here
8.1. Generic Routing Encapsulation on the
A GRE packet is composed of a header in which a type field
the encapsulated protocol (ARP, IP, IPX, etc.). See [RFC2784]
details about the encapsulation. In our case, only support for
MAC addressing scheme of the unidirectional link MUST be implemented
A packet tunneled with a GRE encapsulation has the following format
the delivery header is an IP header whose destination is the
end-point (FBIP), followed by a GRE header specifying the link-
type of the unidirectional link. Figure 4 presents the
encapsulated packet
----------------------------------------
| IP delivery header |
| destination addr = FBIP |
| IP proto = GRE (47) |
----------------------------------------
| GRE Header |
| type = MAC type of the UDL |
----------------------------------------
| Payload packet |
| MAC packet |
----------------------------------------
Figure 4: Encapsulated
9.
9.1. Hardware address
Regardless of whether the link is unidirectional or bidirectional,
a feed sends a packet over a non-point-to-point type network,
requires the data link address of the destination. ARP [RFC826]
used on Ethernet networks for this purpose
The link-layer mechanism emulates a bidirectional network in
presence of an unidirectional link. However, there are
delays between every (feed, receiver) pair. The backchannel
a receiver and a feed has varying delays because packets go
the Internet. Furthermore, a typical example of a
link is a GEO satellite link whose delay is about 250 milliseconds
Duros, et al. Standards Track [Page 17]
RFC 3077 LL Tunneling Mechanism for UDLs March 2001
Because of long round trip delays, reactive address
methods such as ARP [RFC826] may not work well. For example, a
may have to forward packets at high data rates to a receiver
hardware address is unknown. The stream of packets is passed to
link-layer driver of the feed send-only interface. When the
packet arrives, the link-layer realizes it does not have
corresponding hardware address of the next hop, and sends an
request. While the link-layer is waiting for the response (at
250 ms for the GEO satellite case), IP packets are buffered by
feed. If it runs out of space before the ARP response arrives,
packets will be dropped
This problem of address resolution protocols is not addressed in
document. An ad-hoc solution is possible when the MAC address
configurable, which is possible in some satellite receiver cards.
simple transformation (maybe null) of the IP address can then be
as the MAC address. In this case, senders do not need to "resolve
an IP address to a MAC address, they just need to perform the
transformation
9.2. Routing
The link-layer tunneling mechanism hides from the network and
layers the fact that feeds and receivers are connected by
unidirectional link. Communication is bidirectional, but
in bandwidths and delays
In order to incorporate unidirectional links in the Internet,
and receivers might have to run routing protocols in some topologies
These protocols will work fine because the tunneling
results in bidirectional connectivity between all feeds
receivers. Thus routing messages can be exchanged as on
bidirectional network
The tunneling mechanism allows any IP traffic, not just
protocol messages, to be forwarded between receivers and feeds
Receivers can route datagrams on the Internet using the most
feed or receiver as a next hop. Administrators may want to set
metrics used by their routing protocols in order to reflect
routing tables the asymmetric characteristics of the link, and
direct traffic over appropriate paths
Feeds and receivers may implement multicast routing and
dynamic multicast routing can be performed over the
link. However issues related to multicast routing (e.g.,
configuration) are not addressed in this document
Duros, et al. Standards Track [Page 18]
RFC 3077 LL Tunneling Mechanism for UDLs March 2001
9.3.
The DTCP protocol does not generate a lot of traffic whatever
number of nodes. The problem with a large number of nodes is
related to this protocol but to more general issues such as
maximum number of nodes which can be connected to any link. This
out of scope of this document
10. IANA
IANA has reserved the address 224.0.0.36 for the "DTCP announcement
multicast address as defined in Section 7.
IANA has reserved the udp port 652 for the HELLO_PORT as defined
Section 7.
11. Security
Many unidirectional link technologies are characterised by the
with which the link contents can be received. If sensitive
valuable information is being sent, then link-layer
mechanisms are an appropriate measure. For the UDLR protocol itself
the feed tunnel end-point addresses, sent in HELLO messages, may
considered sensitive. In such cases link-layer security
may be used
Security in a network using the link-layer tunneling mechanism
be relatively similar to security in a normal IPv4 network. However
as the link-layer tunneling mechanism requires the use of tunnels,
introduces a potential for unauthorised access to the service.
particular, ARP and IP spoofing are potential threats because
may not be authorised to tunnel packets. This can be countered
authenticating all tunnels. The authenticating mechanism is
specified in this document, it can take place either in the
IP protocol (e.g., AH[RFC2402]) or in an authentication
integrated with the tunneling mechanism
At a higher level, receivers may not be authorised to provide
information even though they are connected to the
link. In order to prevent unauthorised receivers from providing
routing information, routing protocols running on top of the link
layer tunneling mechanism MUST use authentication mechanisms
available
Duros, et al. Standards Track [Page 19]
RFC 3077 LL Tunneling Mechanism for UDLs March 2001
12.
We would like to thank Tim Gleeson (Cisco Japan) for his
editing and technical input during the finalization phase of
document
We would like to thank Patrick Cipiere (UDcast) for his
input concerning the design of the encapsulation mechanism
We would like also to thank for their participation: Akihiro
(IMD), Akira Kato (Tokyo Univ.), Hitoshi Asaeda (IBM/ITS),
Komatsu (JSAT), Hiroyuki Kusumoto (Keio Univ.), Kazuhiro Hara (Sony),
Kenji Fujisawa (Sony), Mikiyo Nishida (Keio Univ.), Noritoshi
(Sony CSL), Jun Murai (Keio Univ.), Jun Takei (JSAT) and
Hakulinen (Nokia).
Duros, et al. Standards Track [Page 20]
RFC 3077 LL Tunneling Mechanism for UDLs March 2001
Appendix A: Conformance and
This document describes a mechanism to emulate
connectivity between nodes that are directly connected by
unidirectional link. Applicability over a variety of equipment
environments is ensured by allowing a choice of several key
parameters
Thus in order to ensure interoperability of equipment it is
enough to simply claim conformance with the mechanism defined here
A usage profile for a particular environment will require
definition of several parameters
- the MAC format
- the tunneling mechanism to be used (GRE is recommended
- the "tunnel type" indication if GRE is not
For example, a system might claim to implement "the link-
tunneling mechanism for unidirectional links, using IEEE 802 LLC,
GRE encapsulation for the tunnels."
Appendix B: DTCP announcement address transition
Some older receivers listen for DTCP announcements on the 224.0.1.124
multicast address (the "old DTCP announcement" address). In order
support such legacy receivers, feeds SHOULD be configurable to
all announcements simultaneously to both the "DTCP announcement
address, and the "old DTCP announcement" address. The
setting is to send announcements to just the "DTCP announcement
address
In order to encourage the transition plan, the "old" feeds MUST
updated to send DTCP announcements as defined in this section.
number of "old" feeds originally deployed is relatively small
therefore the update should be fairly easy. "New" receivers
support "new" feeds, i.e., they listen to DTCP announcements on
"DTCP announcement" address
Duros, et al. Standards Track [Page 21]
RFC 3077 LL Tunneling Mechanism for UDLs March 2001
[RFC826] Plummer, D., "An Ethernet Address Resolution Protocol",
37, RFC 826, November 1982.
[RFC1112] Deering, S., "Host Extensions for IP Multicasting", STD 5,
RFC 1112, August 1989
[RFC1700] Reynolds, J. and J. Postel, "ASSIGNED NUMBERS", STD 2,
1700, October 1994.
[RFC2119] Bradner, S., "Key words for use in RFCs to
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2402] Kent, S. and R. Atkinson, "IP Authentication Header",
2402, November 1998.
[RFC2453] Malkin, G., "RIP Version 2", STD 56, RFC 2453,
1998.
[RFC2780] Bradner, S. and V. Paxson, "IANA Allocation Guidelines
Values In the Internet Protocol and Related Headers",
37, RFC 2780, March 2000.
[RFC2784] Farinacci, D., Hanks, S., Meyer, D. and P. Traina, "
Routing Encapsulation (GRE)", RFC 2784, March 2000.
Duros, et al. Standards Track [Page 22]
RFC 3077 LL Tunneling Mechanism for UDLs March 2001
Authors'
Emmanuel
1681, route des
Les Taissounieres - BP 355
06906 Sophia-Antipolis
Phone : +33 4 93 00 16 60
Fax : +33 4 93 00 16 61
EMail : Emmanuel.Duros@UDcast.
Walid
INRIA Sophia
2004, Route des Lucioles BP 93
06902 Sophia
Phone : +33 4 92 38 77 18
Fax : +33 4 92 38 79 78
EMail : Walid.Dabbous@inria.
Hidetaka
JSAT
Toranomon 17 Mori Bldg.5
1-26-5 Toranomon, Minato-
Tokyo 105
Phone : +81-3-5511-7568
Fax : +81-3-5512-7181
EMail : izu@jsat.
Noboru
Sony
2-10-14 Osaki, Shinagawa-
Tokyo 141
Phone : +81-3-3495-3092
Fax : +81-3-3495-3527
EMail : fujii@dct.sony.co.
Duros, et al. Standards Track [Page 23]
RFC 3077 LL Tunneling Mechanism for UDLs March 2001
Yongguang
RL-96, 3011 Malibu Canyon
Malibu, CA 90265,
Phone : 310-317-5147
Fax : 310-317-5695
EMail : ygz@hrl.
Duros, et al. Standards Track [Page 24]
RFC 3077 LL Tunneling Mechanism for UDLs March 2001
Full Copyright
Copyright (C) The Internet Society (2001). All Rights Reserved
This document and translations of it may be copied and furnished
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or assist in its implementation may be prepared, copied,
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included on all such copies and derivative works. However,
document itself may not be modified in any way, such as by
the copyright notice or references to the Internet Society or
Internet organizations, except as needed for the purpose
developing Internet standards in which case the procedures
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English
The limited permissions granted above are perpetual and will not
revoked by the Internet Society or its successors or assigns
This document and the information contained herein is provided on
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE
Funding for the RFC Editor function is currently provided by
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Duros, et al. Standards Track [Page 25]
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just be content we did not write this in Java, which would have made this "bigger and better" HAHAHHA.
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